1. Field of the Invention
This invention relates to a method and an apparatus for manufacturing castings. More particularly, it relates to a casting method which employs a pattern of the meltaway type and an apparatus which is used for carrying it out.
2. Description of the Prior Art
There are known a variety of casting methods of the nature to which this invention pertains. Some of the known methods will be described, though not shown in the accompanying drawings.
According to a first method, a pattern of the meltaway type which is formed from a polystyrene foam or the like is embedded in a filling material, such as casting sand, in a metallic molding flask. A runner has a lower end connected to the pattern and an upper end provided with a sprue. The flask has a sidewall provided with a multiplicity of vent holes which are so small as to allow the passage of only gas, while virtually no filling material can pass therethrough. The molten metal which is supplied through the sprue and the runner heats the pattern and melts it away. The meltaway of the pattern forms a cavity in which the molten metal is molded. The gas rising from the melted pattern flows through the interstices of the particles of the filling material and leaves the flask through the vent holes.
According to a second method, a flask of the type which is used by the first method is placed in an outer housing which is larger than the flask. The housing has an upper end joined to the upper end of the flask in a gas-tight fashion and defines a gas chamber surrounding the flask. A suction pump is connected to the gas chamber. The gas rising from the melted pattern and leaving the flask, as hereinbefore stated in connection with the first method, enters the gas chamber and is, then, drawn out by the pump.
A third method employs a flask not having any vent hole and a gastight tubular venting device embedded in the filling material and having a sidewall provided with a multiplicity of vent holes. The venting device has an upper end connected to a suction pump. The gas rising from the pattern which has been melted away by the molten metal flows through the interstices of the particles of the filling material, enters the venting device through its vent holes and is drawn out by the pump.
All of the methods which have hereinabove been described have a number of drawbacks. The drawbacks of the first method are due to the fact that no positive means is used for removing the gas, but that the gas is merely allowed to flow out. The gas can be removed only slowly. Therefore, its pressure is likely to cause the molten metal to flow back through the runner and blow out at the sprue and result in the failure to be properly molded. The pressure of the gas is also likely to break or deform the wall of the cavity and thereby make it difficult to manufacture any sound product. Moreover, the gas leaving the flask contaminates its environment.
The second method is uneconomical. It requires a suction pump having a large capacity. It also requires a member for connecting the pump and the gas chamber. This method is difficult to carry out if a plurality of flasks are disposed in a circular array so as to be rotatable in a horizontal plane for receiving a molten charge at one station and delivering a cast product at another station. It is a troublesome job to disconnect the pump from the gas chamber of one flask and connect it to the gas chamber of another flask.
The third method also requires a suction pump having a large capacity and a member for connecting it to the venting device. It is difficult to carry out if a plurality of flasks are employed as hereinabove stated in connection with the second method.